Several series of new dimeric analogs of enkephalin penta-, tetra- and tri-peptides have been synthesized, purified, and characterized by several physical chemical techniques, receptor assays, bioassays, and in vitro bioassays. The new analogs show extremely high potency and specificity for the "delta" opiate receptor, and are useful in the characterization of opiate receptors, auto-radiographic and pharmacological applications. Binding kinetics indicate that the dimers show an increased rate of association and decreased rate of dissociation. Effects of ions and other perturbations of binding of monomers and dimers have been analyzed. The ability of the dimers to induce desensitization and tolerance in NG108-15 cells has been characterized, and may differ qualitatively from that of the monomers. Additional new compounds include alky lenkephalin amides and mono-N-acetyl enkephalin. The latter compounds provide a novel probe of the delta receptor. The potency of these compounds are strongly supportive of the hypothesis that the dimers bind to two sites simultaneously. This provides insight regarding the nature of the organization of the receptor in the member. Methods to alter membrane fluidity are under study.